The present invention relates to an apparatus for distributing a liquid and a vapor to or from a packing in an exchange column for heat and/or mass transfer processes. The apparatus has particular application in cryogenic air separation processes utilizing distillation, although it may be used in other heat and/or mass transfer processes that use packing (e.g., random or structured packing).
The term, "column", as used herein, means a distillation or fractionation column or zone, i.e., a column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements or on a series of vertically-spaced trays or plates mounted within the column.
The term "packing" means solid or hollow bodies of predetermined size, shape, and configuration used as column internals to provide surface area for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of two phases. Two broad classes of packings are "random" and "structured".
"Random packing" means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. Random packings are small, hollow structures with large surface area per unit volume that are loaded at random into a column.
"Structured packing" means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of expanded metal or woven wire screen stacked in layers or as spiral windings; however, other materials of construction, such as plain sheet metal, may be used.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen). Various packings or trays may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases.
The use of packing for distillation is standard practice and has many advantages where pressure drop is important. However, packed column performance is very dependent on creating and maintaining a balance between the downward flow of liquid and the upward flow of vapor locally in the packing. The distribution of the liquid and the vapor within the packing is influenced by the initial presentation of these fluids to the packing.
Initial presentation of liquid and vapor to the packing is usually made by means of distributors. A liquid distributor, the role of which is to irrigate the packing uniformly with liquid, is located above the packing, while a vapor distributor, the role of which is to create uniform vapor flow below the packing, is located below the packing.
In practice, packed distillation columns can be separated into two or more packed sections. The number of packed sections depends on the particular separation being carried out. For example, each section may have a unique gross ratio of the molar flow of liquid and vapor ("L/V ratio") to perform the distillation most efficiently. In this case, liquid and/or vapor may be transported to or from another column, tank, or heat exchanger and withdrawn or fed above and below each packed section. Another required use of separate packed sections arises when the packed section height approaches some maximum value, above which it is necessary to collect and/or redistribute the two fluids before continuing the separation. The redistribution of the fluids acts to mitigate the effects of maldistribution. In this case, a single tall section, for example, would be divided into two or more shorter sections, each with substantially the same gross L/V ratio.
The use of a multiplicity of separate packed sections in a column requires a multiplicity of liquid and vapor distributors above and below each section. Adjacent distributors typically work in tandem; that is, the vapor distributor distributes vapor while also collecting liquid from the section above and passing it to the liquid distributor below. Feeds and draws of liquid and/or vapor between the packed sections are made in connection with the distributors as well. For example, a feed of liquid may be made through the side of the column, directly into the liquid distributor which simultaneously receives liquid collected by the vapor distributor above.
Besides the role of gross flow distribution, distributors located between packed sections also can mitigate the effects of gross concentration gradients in a column. The mixing of liquid in the vapor and liquid distributors collected from different radial locations in the packed section above reduces any composition differences across the column cross-section. The liquid distributor may also mix liquids fed to the column from a sidefeed with liquids collected from the section above. The vapor distributor can act in a similar way to mix vapors across the column cross-section and to mix vapor introduced from outside the column with the predominant upward-flowing vapor in the column.
Offsetting the advantages afforded by liquid and vapor distributors to column performance, each distributor also adds significant height to the column. The vertical distance between the bottom of a packed section and the top of an adjacent packed section below it must be large enough to accommodate the vapor distributor, the liquid distributor, and any feed-nozzles or draw-nozzles. Vertical distance is needed below the liquid distributor and above the vapor distributor to give the vapor flow sufficient disengagement space so as not to affect flow uniformity in the packing. Some additional vertical distance may be necessitated by the requirements of installing the individual distributor components and nozzles into the column.
In addition to the cost of column height, each distributor also incurs a fabrication cost. Besides the height requirements and fabrication costs of vapor and liquid distributors, their close proximity also adds complexity and cost to column design.
For example, the vapor distributor must be designed with regard to the orientation, geometry, and proximity of the liquid distributor and the feed/draw nozzles to avoid severe vapor maldistribution from the "shadowing effect" of these components on the vapor flow. There is an added vertical distance requirement which arises from locating the distributors adjacent to each other. Vertical distance is needed between the liquid distributor and the vapor distributor for additional disengagement space necessitated by vapor flow nonuniformity caused by the liquid distributor before the vapor passes into the vapor distributor.
Thus, optimum packed column liquid and vapor distribution systems effectively distribute both liquids and vapors with minimum column height requirements and fabrication costs while promoting adequate mixing of the individual fluids.
There are three main types of typical liquid distributors--pipe, pan, and trough distributors. Each type is discussed briefly below.
Pipe distributors are comprised of an interconnecting network of closed pipes or ducts, typically comprising a central pipe or manifold and a number of arms or branches radiating from the central pipe. The arms are perforated to allow the liquid passing from the central pipe and into the arms to be dripped or sprayed onto a packed bed below the pipe distributor. Upwardly flowing vapor passes easily in-between each arm. Pipe distributors receive liquid from a separate liquid collector or an external source piped through the wall of the column. While simple and inexpensive to construct, pipe distributors may distribute liquid poorly when vapor gets trapped in the arms.
Pan distributors are comprised of a pan or pot, having holes in the bottom for feeding liquid to the packing below, and tubes or risers for the vapor to pass upwardly through the distributor. Pan distributors often make a complete seal with the wall of a column. Thus, pan distributors can act as liquid collectors as well as distributors. However, since large pan distributors are costly to build, pan distributors usually are used in smaller columns, i.e., columns with diameters less than 1.5 meters.
Trough distributors comprise a collection of interconnecting open troughs having irrigation holes in the base to feed liquid to the packing below. One or more upper collection troughs, or a simple pot on top of the lower troughs feeds liquid to the lower troughs through a series or holes or overflowing notches. Vapor from the packing below passes upward between the liquid-containing troughs.
Initial presentation of vapor is made by a vapor distributor, which typically comprises a device which imparts a flow restriction on the vapor, such as a perforated tray or series of small diameter "chimneys" mounted on a flat plate which is otherwise sealed to vapor flow. The flow restriction causes a pressure drop in the vapor as it passes through the device. The imposed pressure drop is made to exceed any radial pressure gradients which are likely to exist within the vapor phase, and thereby forces vapor-flow-rate uniformity across the cross-section of the column.
Attempts have been made to reduce the column height between two packed sections by combining two or more of the following--liquid collection, liquid distribution, and vapor distribution--into one or two pieces of equipment. These fall into two categories: 1) combined liquid collectors/liquid distributors; and 2) combined liquid collectors/liquid distributors/vapor distributors. (It is noted that the typical vapor distributor, comprising a perforated tray and a downcomer(s), also acts as a liquid collector.)
A pan-type liquid distributor can act as a liquid collector below a packed section as previously mentioned, provided that the vapor risers are shielded from the liquid dripping down from the packing above. Ways have been suggested by which trough distributors can act as liquid collectors/distributors by shielding vapor risers. These are a type of re-distributor, because they typically do not accept a liquid feed.
Variations on the combined liquid collector/distributor have been proposed. For example, in one design, a series of troughs is placed at two or more elevations, so that the upper layer of troughs is staggered with respect to the lower series of troughs, thus forming a tortuous path for the vapor and attempting to collect all the liquid raining down from the packing above. A wall seal is made by a special deflector sheet at each elevation.
In another design, a series of nested, truncated funnels placed over a trough-type liquid distributor act as a liquid collector. Gaps between each funnel allow vapor to pass upward between the funnels. The diameter of the top of each funnel is larger than the bottom of the adjacent funnel, so that the liquid coming from the packing above is collected.
In yet another design, a series of strips placed over vapor risers deflect liquid raining down onto a trough-type liquid distributor. This design can be used in conjunction with a liquid feed nozzle whose perforations are located over the strips, so that the feed liquid can be mixed with the liquid raining down on the strips as it spills into the troughs.
Attempts at promoting liquid mixing within distributors by various means have been suggested in the prior art. In U.S. Pat. No. 5,240,652, a pan-type distributor with capped risers acts as a liquid collector and feeds liquid to a central trough, which then distributes the liquid to a lower pan distributor. The central trough acts as a predistributor for the lower pan distributor and also mixes the liquid collected on the liquid collector. This type of distributor has several disadvantages, including the fact that the liquid collection and pre-distribution functions are performed at two elevations. Also, this type of distributor does not provide for liquid or vapor feeds and/or draws.
In European Patent No. EP0782877A1, a baffle is placed inside a pan re-distributor to promote mixing within the liquid reservoir by forcing the liquid to follow a tortuous path inside the distributor.
Perhaps the most significant attempts at combining distributor functions are disclosed in Canadian Patent No. 2,173,280 and U.S. Pat. Nos. 5,132,055 and 5,224,351 (and its equivalent, European Patent No. EP0644144A2). These patents disclose combined distributors which act as liquid collector/distributors, vapor distributors, and structured packing support structures. Variations accept liquid and vapor feeds and draws, perform internal liquid mixing, and act as transition elements between column sections of different diameters. The basic unit of the disclosed combined distributor is a trough distributor having vapor risers formed from inverted U-shaped elements which have narrow slots located on the sloping portion of the riser for vapor to pass through. The packing rests on the top of the inverted U-shaped risers.
There are several disadvantages with those and other prior art distributors. For example, the distributors do not reduce any concentration gradients that may have developed across the column in the vapor phase. The distributors also are susceptible to malperformance caused by gross liquid maldistribution in the packed section above the distributor. In some of the designs, the troughs receive liquid directly from the packing above. Any non-uniformity in the received liquid flux must be dampened by hydrodynamic resistance of the orifices in the bottom of the troughs; dampening is accomplished primarily by decreasing the number and/or area of the orifices in the bottom of the troughs to increase the liquid level in the troughs, so that the liquid level variations in the troughs caused by flux non-uniformity are small relative to the total liquid head in the troughs. Increased liquid level requires increased trough height, which, in turn, requires increased column height. At very high levels of liquid maldistribution, the additional column height required to achieve a liquid level in the troughs adequate to dampen the flux non-uniformity becomes excessive.
The prior art designs which utilize baffles, such as European Patent No. EP0782877A1, impose an added hydrodynamic resistance to liquid migration within the troughs, and therefore negatively affect the distribution when liquid maldistribution is present in the packed section above the combined distributor.
It is desired to have a combined vapor/liquid distributor for packed columns which affords better liquid and vapor mixing than the prior art liquid distributors and vapor distributors, and which also overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It is further desired to have an optimum combined vapor/liquid distributor which effectively distributes both liquids and vapors with minimum column height requirements and fabrication costs while promoting adequate mixing of the liquids and vapors.
It is still further desired to have a combined vapor/liquid distributor for packed columns which provides for liquid and/or vapor feeds and/or draws.
It is still further desired to reduce cross-column concentration gradients.
It is still further desired to decrease the likelihood of malperformance caused by gross liquid maldistribution in packed columns by mitigating the effects of liquid maldistribution.
It is still further desired to have a combined vapor/liquid distributor which performs both liquid collection and pre-distribution functions at a single elevation within the column, and which also distributes vapor and provides the option for liquid and/or vapor feeds and/or draws.
It is still further desired to have a combined vapor/liquid distributor that shows high performance characteristics for cryogenic applications, such as those used in air separation, and for other heat and/or mass transfer applications.
It is still further desired to have an optimal design of a combined vapor/liquid distributor that operates in an optimal manner and minimizes its size, weight and/or cost, which will result in an air separation process more efficient and/or less expensive per unit quantity of product produced.
It is still further desired to have a more efficient air separation process utilizing a combined vapor/liquid distributor which is more compact and more efficient than the prior art.
It also is further desired to have a method of assembling a combined vapor/liquid distributor for exchange columns which affords better liquid and vapor mixing than the prior art liquid distributors and vapor distributors, and which also overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It also is further desired to have a new, more efficient method for the collection and distribution of a liquid and a vapor in exchange columns.